The invention relates generally to color image display systems, and particularly to a system associating a deflection yoke assembly with a shadow mask type cathode-ray tube incorporating external shield means disposed along opposite sides of the tube and across the longitudinal axis behind the tube, while providing access to the deflection yoke assembly and the neck components.
In a color image display system having a shadow mask type cathode-ray tube, a plurality of convergent electron beams are projected through a multi-apertured color selection shadow mask to a mosaic phosphor screen. The beam paths are such that each beam impinges upon and excites only one color of the color-emitting phosphor of the screen.
When a shadow mask type cathode-ray tube is operated, external magnetic fields, such as the earth's field or fields created by the electronic components of the image display system, will cause electron beam path distortion and thereby affect the register of the electron beams with the phosphor elements of the screen. The exact misregister caused by the earth's magnetic field varies substantially in widely separated geographic locations and depends upon the orientation of the tube in the magnetic field and the size of the tube. For example, in computer display terminals and home television receivers, the tube is generally oriented so that the longitudinal axis of the tube is horizontal; however, in some video game applications, the tube may be placed in a table so that the longitudinal axis is directed upward. In the United States, the average intensity of the horizontal component of the earth's magnetic field is about 20,000 gamma, whereas the average intensity of the vertical component is about 47,000 gamma. Thus, in the United States, a shadow mask type cathode-ray tube, which is oriented so that the tube axis is aligned parallel to the vertical component of the earth's magnetic field, experiences greater misregister of the electron beams than if the same tube were operated with the tube axis parallel to the horizontal component of the earth's magnetic field. Larger diameter tubes show a greater effect because of the longer beam paths. Tube operation in different parts of the world also must take into consideration the relative strength of the two components of the earth's magnetic field at the geographical point of operation. By way of example, the horizontal component of the earth's magnetic field intensity in Singapore is about 40,000 gamma or twice that of the United States. The use of magnetic shielding to minimize these magnetic effects is well known; however, a satisfactory solution to the problem has not yet been achieved.
Conventional magnetic shielding is shown in U.S. Pat. No. 3,867,668, issued to Shrader on Feb. 18, 1975. In the Shrader patent, a two portion external shield extends partially around the funnel of the tube and overlies an opening in the internal shield. The structure of the Shrader patent does not provide shunting of the magnetic field component directed parallel to the longitudinal axis of the tube. The use of fully enclosing external shielding to eliminate these magnetic effects is not feasible. To be effective, such a shield would have to project well beyond the front of the tube to prevent the field from entering through the faceplate of the tube. Furthermore, in a shield of any practical size, its effect in distorting and concentrating the earth's field at particular points might be more serious than the reduction in average field strength. Additionally, initial yoke assembly adjustments are made with the external shielding removed from the tube. The installation of the external shielding frequently causes changes in the adjustment and operation of the tube. Fully enclosed external shielding therefore requires openings or other means which provide access to readjust the yoke assembly and neck components. Such fully enclosed shields, with accurately located openings or access means, are costly and impractical for all but the most demanding applications. An example of a "fully enclosed" tube may be found, for example, in U.S. Pat. No. 3,404,227, issued to Alcala et al. on Oct. 1, 1968. In the Alcala et al. patent, an aluminum housing, which encircles the funnel and a portion of the neck of the tube, is sprayed with a magnetic shielding material in the funnel portion of the housing. A cap or end cover encloses the base of the tube; however, the magnetic shielding structure does not extend behind the neck of the tube and across the tube axis and, therefore, does not shunt the component of the earth's field parallel to the longitudinal axis of the tube. Another example of a "fully enclosed" tube in which a magnetic shield encases all the cathode-ray tube components and the tube itself is found in U.S. Pat. No. 3,887,766, issued to Caswell on June 3, 1975. The Caswell shielding structure, comprising a double walled cylindrical housing, which extends beyond the neck of the tube, is a complex structure that does not provide ready access to the deflection assembly and neck components. Such a structure is expensive to produce and makes it difficult to adjust the focus coil and deflection yoke when the shielding structure encloses the tube, since the outer shield does not provide access to the coil or yoke.